Hi
Conventional boiler & separate cylinder
2 x 2 port zone valves
Stat works
Programmer works
HW fires up the boiler
but boiler won't fire up when CH is demanded.
When CH is turned on (and the HW is already turned on) then the rads get hot.

I reckon I know what the fault is but would appreciate your diagnosis.

the output from the valve is the orange wire, which has to go live to fire the boiler. Put it into a strip con by itself and see if it goes on.
This is the output from the microswitch as everyone else has suggested.

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----------------------------------------Lack of planning on your part doesn't make it an emergency on mine....

Depending on the valve head, you can sometimes take the cover off and get access to the micro switch to switch it manually. Sometimes the valve works, and the switch works, but the little rubber balls inside the vave body can deform so it doesn't quite go right over, or bits of grit can get caught and cause the same issue.

Well I was wrong. Having ruled out y plan (3 port valve), I had assumed 2 x 2 port zone valves.
Anyway, there were no valves at all. The HW is gravity fed, from the back boiler to the cylinder, and the central heating is just a pump controlled by the room stat.
Pump had failed.
Old, & Odd.

I'm not sure if that's better or not than the feeling I had of just being old (again) being able to remember such things first hand. My excuse is that I started taking things apart to see how they worked at a very early age, and I'm sticking to it!
- Andy.

Sounds like you started the same way as I did Andy! I suppose it's because we didn't have all the toys and gadjets that kids have now, eg Playstations, computers phones etc,etc? (I think we must be from a similar era)

It sounds like it! for me it was an era of make-do-and-mend, and when it can't be mended - given to Andy to take apart! I still remember vividly getting into awful trouble after being given an old clothes iron, taking it apart outside and finding it full of hundreds of sheets of mica, which then promptly blew around the garden and lodged in every tree and bush in the neighbourhood. I must confess I did have a computer though - it came as a kit of components and empty circuit board and I had to spend 2 solid days soldering it all together!